A woman and her granddaughter are waiting to speak to the “famous” professor “who solves everything.” Professor T is already familiar with their story: the woman’s daughter (and granddaughter’s mother) Beverly has been murdered by blows from a hammer. A drug addict named Kane Carty is in custody, having been caught trying to sell Beverly’s watch. But Sophie, Beverly’s daughter, says Carty isn’t the murderer.
The 18-year-old Sophie is autistic and has Down syndrome. She was discounted as an unreliable witness by the police weeks ago, and only wants to speak to Professor T. But she won’t reveal the true murderer—the professor believes he must win her confidence and ask the right question for her to share that information.
Lisa disagrees with the decision to trust Sophie and reopen the case, but she begrudgingly interviews Carty again. He was smoking in the woods when he thought he saw someone, who quickly vanished. He stumbled over Beverly’s body, thinking she was a fellow addict, unconscious, and took her watch and handbag. According to him, her watch was already undone—which Professor T thinks suggests an attempt to disguise the murder as a robbery. He also points out that Carty was smoking heroin, which makes one euphoric, not violent or angry, making it unlikely that he would have attacked a random person.
Sophie was at home with her grandmother at the time of the murder. They saw Beverly leave to catch a bus to her sister Veronica’s home; the woods where she was found were a shortcut. Beverly’s husband, Andrew, has a solid alibi: he had a business meeting, otherwise he would have driven Beverly to her sister’s.
While the police re-interview Veronica, Professor T asks her about the plan to put Sophie into assisted living—information he discovered from rifling through Andrew’s mail. Sophie has lately become more challenging, even aggressive, Veronica says, and so has become too much for Beverly. Professor T finds Veronica performative, and gets Christina to assign surveillance on her.
Dan volunteers himself and Lisa for the job, so that he can spend time with her. She apologizes for not responding to his texts asking to see her—she has a lot on her plate. Her ailing father’s dementia is so bad that he has confused Lisa for her mother, and asked her to marry him. But Lisa likes Dan, and proposes they go on a proper date.
They see Veronica meet Andrew in a park, and kiss him.
When confronted, Veronica says that Beverly didn’t know about the affair, which has gone on for about a year. Andrew and Beverly weren’t happy; she constantly belittled him and called him a loser for not making enough money to support them, while she had little contact with the outside world, as caring for Sophie took up most of her time. When Beverly didn’t show up at her house the night she was murdered, Veronica tried calling her and then the house, where Sophie picked up and said that her grandmother was asleep.
Veronica’s husband throws her out, and tells the detectives to focus on Andrew, who said he would do anything to keep Sophie at home.
Andrew didn’t want to send Sophie to assisted living—it was too expensive and he loves having his daughter around. He and Beverly argued over it; she smashed a teapot. Sophie tells Professor T that she didn’t want to leave her home either. The prospect agitated her enough that her parents began giving her medicination to calm her.
Lisa has begun to suspect that Sophie lashed out and accidentally killed her mother, and her father helped hide the evidence. But the professor won’t let her interview Sophie again. And a forensic test of Andrew’s car is clean, meaning he didn’t transport the body.
Then Sophie goes missing, to Andrew’s distress. Lisa suspects something fishy, and calls Christina to ask if the professor has anything to do with Sophie’s disappearance. Christina assures Lisa that Sophie is safe and can make her own decisions. She came of her own volition.
Sophie’s grandmother receives the same assurance, and tells Professor T that she was planning to help pay for Sophie’s assisted living. Professor T in turn asks Sophie, who is being watched by Adelaide and Mrs. Snares, why her grandmother was asleep so early the night Beverly died. Sophie gave her the same medication used to calm her; she didn’t want to watch her grandmother’s boring soap operas.
Lisa lashes out at Christina for allowing Professor T to take Sophie away from Andrew. You don’t know what it’s like to care for someone completely reliant on you, she yells. Then the professor appears with Sophie and asks her if her dad killed her mom. She nods, then hugs the professor. He smiles at the embrace.
The hammer used to kill Beverly is found in a pond at Andrew’s house. He explains that Beverly always belittled him even though he worked hard; he felt she used Sophie as an excuse to avoid a job, and then was going to give up even that by sending her away. She missed her bus that night, and Andrew drove her to her Veronica’s. But they hit something, and when he got out to check the car with his toolbox, Beverly harangued him. He snapped; he just wanted her to shut up after so many years. He kept his meeting but stopped at home first to clean up and hide the hammer. That’s when Sophie saw him.
Lisa apologizes to Christina for being wrong about so much in the case. Christina, having observed Lisa and Dan’s relationship, offers to let her transfer.
Instead of showing up at her date with Dan, Lisa sends a friend. Dan goes to the bathroom and vents some anger, then rinses his face and goes back out to Lisa’s friend to begin anew.